Field of the Invention
The present invention relates to machine tools such as lathes capable of performing, e.g., a threading process.
Description of Related Art
When machining is performed with a machine tool (in particular, machining of a workpiece with low rigidity or machining using a tool with low rigidity), strong vibrations known as chatter vibrations are often generated during machining. Such chatter vibrations cause problems such as a so-called chatter mark on the cutting surface, chipping of the tool, etc. As a solution to such problems, a technique has been devised which suppresses the chatter vibrations by changing the main spindle rotation speed during a threading cycle, as disclosed in, e.g., Japanese Patent Application Publication No. 2004-209558 (JP 2004-209558 A).
The above conventional techniques show a method of varying the main spindle rotation speed, a method of changing the main spindle rotation speed for a finishing process, and a method of changing the main spindle rotation speed in every pass as examples of the method of changing the main spindle rotation speed. The conventional techniques, however, do not show any appropriate method of changing the main spindle rotation speed. If a cutting method of the tool passes (hereinafter referred to as a “cutting mode”) is a so-called alternate flank infeed mode, the factors that significantly affect the current cutting is the cutting surface of two passes before. Accordingly, effective suppression of the chatter vibrations cannot always be expected by using the main spindle rotation speed changed according to cutting of one pass before. Thus, when changing the main spindle rotation speed, the operator needs to decide setting of the number of passes in which cutting is continuously performed at the same main spindle rotation speed (hereinafter referred to as the “number of same rotation speed cutting passes) each time according to the cutting mode etc. It is therefore not easy for unskilled operators to use such machining tools. Prior to the present application, the applicant has devised a technique capable of suppressing an increase in cutting load. In the technique, the same run-out angle is used in all the passes when the main spindle rotation speed is changed on a pass basis during a threading cycle (Japanese Patent Application No. 2012-121107). The present invention has been developed in view of the technique.